Many products are assembled from components that have similar outward appearances. When these products are assembled, there is a class of errors that result from the assembler assembling the components in the wrong order. In many cases, these errors lead to products that perform incorrectly. In some cases, the errors can be detected during a testing phase of the final product, and the defective products returned to the assembly plant for repair. In other cases, the resultant assembly errors lead to intermediate failures that are not always detected when the product is tested.
For example, consider an electronic product in which a large number of printed circuit cards are plugged into a bus and all of the card connectors on the bus are identical. Hence, any card can fit in any slot. In many cases, the cards are also similar in size and appearance to one another. If any card will function in any slot, this does not pose a problem. However, in some cases, there are restrictions as to the locations of some of the cards. For example, two of the cards may need to be placed in adjacent card slots, or one or more of the cards will only function properly if placed in certain slots. If a placement error occurs, the error may not be detected until the final product is tested. In some cases, the failure resulting from misplacement of the cards will not be detected immediately, because the misplacement leads to intermittent failures in the final product. For example, the final product can have a race problem that only appears when the product has been running for an extended period of time or when other activities on the bus are present.
In principle, such placement errors can be avoided by including some form of interlock or identification mechanism on the printed circuit cards and card connectors. For example, each card could include an identification code that can be read by a test system connected to the bus. In such a system, each card slot would also need to be addressable in a manner that allows the test system to determine the identification code of the card in that slot. The test system could then identify misplaced cards. Unfortunately, such a system can substantially increase the cost of the final system.
For example, if the cards and connectors each have a computer readable identification number, a test program can match the cards against the connectors. However, in many cases, the back plane on which the card connectors are mounted does not include the capability of uniquely identifying each connector in a manner that can be read by a test program. Furthermore, the cost of providing such electronically readable identification on each card can also be significant, particularly in systems in which the same card can be used in different products at different locations. In the latter case, some form of jumper is typically used to set the card identification. The jumpers are internal to the cards and not easily readable by an assembly technician. Furthermore, an error in setting the jumper can also lead to products being returned from the testing facility because of an apparent assembly error resulting from a misplaced jumper. In addition, it should be noted that this solution is only possible in the case in which the assembler of the product using the cards also controls the manufacture of the cards and the bus. If one or more of the critical cards are purchased from an outside source, such modifications could be prohibitively expensive, since they would require the outside vendor to modify its production line.
In other cases, the components may not have any form of easily readable identification system to utilize in testing the assembly of the components. For example, non-electrical products having a number of similar mechanical components that are assembled into a finished product lack the ability to apply the above-described testing scheme.